The increasingly sophisticated information processing needs of today's work environment include the concept of an integrated desktop, from which users can access a variety of computer systems having various network architectures. Examples of different computing systems and associated network architectures include UNIX systems using Transmission Control Protocol/Internet Protocol (TCP/IP), Unisys A-series systems using Burroughs Network Architecture (BNA), and IBM mainframes using Systems Network Architecture (SNA).
In such a network environment, there is a need to make the user's access to the various computing systems transparent. In other words, the user should have access to any of the computing systems without the need to reconfigure his integrated desktop (e.g., PC). For example, users may want to move seamlessly from an A-series Menu-Assisted Resource Control (MARC) session to U6000 E-mail, or from a UTS session to a spreadsheet accessing data files on a NetWare server.
With PC platforms such as Microsoft Windows.TM., the integrated desktop is a reality, and consequently the interoperability of network architectures is a fundamental requirement. Clearly, a coherent strategy, along with the appropriate system support, for communicating with all of the computing systems in an integrated environment is desirable.
Part of this strategy may include local area networks. Workstations connected via local area networks have become increasingly important in recent years because they offer computer users shared access to common resources such as storage, input/output, communication devices, etc. When the access to the common resources are beyond the reach of the local area network, a communication server or relay is often employed because a resource is shareable only among systems with identical protocol "stacks". Thus, the communication server performs the necessary protocol translations, depending upon the network layer at which the communication is occurring, to allow internetwork (or inter-protocol) communication.
As background to the possible network layers in which a relay may operate, FIG. 1a illustrates a taxonomy for describing LAN interconnection. The taxonomy associates a LAN interconnection device with an ISO Reference Model layer. Each device is associated with the layer in which it relays information from one network to another. The term network in this context ranges from LAN segments, satellite links, and terrestrial lines in the lower layers to network architectures (e.g., OSI and SNA) in the higher layers.
In this taxonomy it is important to note that the layer performing the relay does not utilize information from the higher layers. In fact, differing higher layer protocols can concurrently utilize the same lower layer relay. Generally, the higher the relay layer, the more specialized are the set of products and protocols serviced by the relay. Also, factors such as overhead and complexity increase with higher layer numbers.
The relay is generally termed a bridge if it operates at the data link layer (which typically comprises two sublayers: logical link control (LLC) and media access control, (MAC) layers). A bridge connects data links for the purpose of forwarding packets between local networks, thereby effectively forming an extended local network.
If the relay operates at the network layer it is generally called a router. A router differs from a bridge in that a bridge generally operates transparently to the communicating end stations while a router is explicitly addressed by end stations requiring routing services. And, if the relay operates at any of the higher layers it is generally known as a gateway.
FIG. 1b illustrates the data flow through respective network layers from a source node to an intermediate node to a destination node. The addition or removal of header (or trailer) data is noted to illustrate the transformation of the data packet which may take place at each network layer, thus, preparing it for the next layer (whether up or down).